The first is http://www.animepaper.net/, an anime wallpaper community that is stunning in its artistry. You'll find countless wallpapers and high-res scans that will take your breath away. Only thing about this site is that you'll need to either contribute your own art, or else contribute some $, in order to have much access to it. Best money I ever spent, personally.

The other link is for the NuonSoft Wallpaper Cycler (http://www.nuonsoft.com/wallpapercycler/). If you're like me and have a couple hundred wallpapers, or even only a couple dozen, this program is by far the best way I have found to manage them. You'll be able to set wallpapers to change in any interval from 1 second to 60 years (!), split them up into categories, set certain categories to only cycle during certain times, set hot keys to instantly change wallpapers, set a stretch that doesn't distort the aspect ratio, add wallpapers recursively through folders... and lots of other cool features. I know this sounds like an ad, but I really couldn't live without this program. It's really, really excellent, and there's a FREE version! Get your booty over to the site and download this thing!